This invention relates to improved seals for alkaline galvanic cells.
Seals for galvanic cells have generally been made from nylon, polypropylene or polysulfone. In the case of alkaline galvanic cells, nylon has been highly preferred, especially nylon 66. However, nylon seals for alkaline galvanic cells have major disadvantages. First, nylon absorbs moisture making it susceptible to hydrolytic degradation in a corrosive electrolyte. As a result of the tendency for nylon to absorb moisture, it must be dried prior to molding. After molding, the dimensions and properties of the resulting seal are affected by the tendency of nylon to absorb moisture. Hydrolytic degradation of nylon occurs through chain scission of amide bonds. Chain scission embrittles the material leading to seal failure and leakage of the cell. To overcome this, protective coatings are sometimes used on the internal side of the seal. In particular, nylon seals for alkaline galvanic cells are generally provided with an coating of asphalt. Application of the asphalt coating involves additional steps and materials which increase the overall cost of the seal.
Another problem with nylon seals for alkaline galvanic cells is that they have a relatively high ultimate elongation. For safety reasons, seals for alkaline galvanic cells are designed to provide controlled release of pressure in the event that the internal pressure of the galvanic cell increases beyond an acceptable limit. This is achieved by forming the seal with a relatively thin portion which is designed to rupture if the internal pressure of the galvanic cell increases beyond an acceptable limit. Sufficient space must be provided within the cell to allow the thin portion to extend and rupture. Under normal moisture conditions, nylon extends over 300% of its initial length. This high level of elongation requires large amounts of internal cell space which limits the seal and cell design. Accordingly, it would be desirable to utilize a material for seal construction which meets the necessary physical and chemical requirements for use as a seal material in an alkaline cell, and which has a relatively lower ultimate elongation.
Another disadvantage with nylon galvanic cell seals is that the physical properties of the seal are dependent upon the moisture content of the nylon. In particular, the strength of a nylon seal is dependent upon its moisture content. The moisture content of the nylon seal is dependent upon the relative humidity of the environment in which it is stored. Accordingly, the vent pressure (i.e., the pressure at which the thin portion of the seal ruptures) of nylon galvanic cell seals is undesirably dependent on relative humidity.
Galvanic cell seals made of polypropylene are subject to extensive softening at the high end of possible use temperatures, i.e., 75-85.degree. C. This softening results in lower deflection temperatures under load and excessive stress relaxation in the compressive sealing zones of the seal and hence leakage of electrolyte and unreliable cell performance.
The use of polysulfone as a material for making galvanic cell seals has been relatively limited on account of its relatively high cost (approximately 2.5 times the cost of nylon 66). In addition to its relatively high cost, polysulfone also has a tendency to absorb moisture, and must be dried to a moisture content of less than or about 0.02% before it can be molded into a seal. This extra step of drying polysulfone before it can be molded further increases the overall cost of forming a galvanic cell seal from polysulfone.